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In a nutshell:
WinForms is based on GDI+, while WPF is based on DirectX.
GDI was used before XP, while GDI+ is used in XP and above.
*Vista and Win 7 actually use a completely different drawing system (The Desktop Window Manager) that buffers GDI+ into a composite image with DirectX to render the operating system.

So yes, and no. Yes, you are using GDI+ to add text to your image. But when it gets displayed on the screen it's actually using DirectX because you are running Win 7.

Don't think you can make the code much faster.

Instead of FillRectangle(), you can do this to paint your bitmap a solid color:

I have a few other questions on this topic right now. If you know any EE gurus on SharpDX please send them my way, okay? Any C# / XNA / game developer types you are acquainted with might know about SharpDX.